Various apparatus have been devised to protect sensitive semiconductor components on printed circuit boards from damage caused by electrostatic energy. For applications requiring precise spark break-in voltages, discrete spark gap components resembling vacuum tubes are sometimes used. For other applications requiring less precision, it is less expensive to form a spark gap directly on the printed circuit board by providing a ground trace in close proximity with a signal trace, separated by an air space. The latter type of spark gap will be referred to herein as a "printed circuit spark gap."
One problem that has been encountered with printed circuit spark gaps is that, in order for them to work properly, the printed circuit traces opposing the gap must be relatively clean (i.e., free from contamination). In manufacturing, this requirement of printed circuit spark gaps must be reconciled with the requirement of applying a protective external coating to printed circuit boards that are intended for use in field environments, Throughout this disclosure, the application of such protective external coatings to printed circuit boards will be referred to as "overcoating."
One such protective overcoating technique is to cover the finished board with a conformal coating. Another technique is to overmold the finished board with a rubber-like elastomeric material. The latter technique is used not only to provide protection against moisture and other environmental conditions to which the board may be exposed during use, but is also sometimes used to create elastomeric structures that are helpful in securing the finished board to a frame or enclosure. Unfortunately, the materials used to create such protective external coatings can interfere with the operation of the printed circuit spark gap.
One prior technique that attempted to produce an elastomer-overmolded printed circuit board having a contaminant-free spark gap is illustrated in FIGS. 1-3. Two copper printed circuit traces 112, 114 were formed on an epoxy-fiberglass composite substrate 100 separated by a narrow space 118. A layer of soldermask 116 was applied to the printed circuit board, but was left open in aperture 117 containing spark gap area 118. A 0.050 inch diameter pin (hereinafter "shutoff pin") 120 was pressed down onto the assembly as shown at arrow 126. A support pin 124 was pressed against the assembly from the opposite side, as shown at arrow 128, to counteract the force of shutoff pin 120. Once shutoff pin 120 and support pin 124 were in place, elastomer was applied to the entire assembly. Typically, the elastomer was applied under high pressure, between 100 and 300 p.s.i. The intention was that the presence of shutoff pin 120 would prevent intrusion of elastomer into aperture 117 and spark gap area 118 during molding. Unfortunately, the profile of soldermask layer 116 over printed circuit traces 112, 114 caused a thin (approximately 0.001 inch) wide gap 130, 132 around the perimeter 122 of shutoff pin 120. The high-pressure elastomer easily infiltrated under shutoff pin 120 into gap 130, 132, causing contamination of aperture 117 and spark gap area 118.
Although theoretically it might be possible to reduce the elastomer intrusion problem by more perfectly aligning the perimeter of shutoff pin 120 with aperture 117, such a solution would require unfeasibly tight mechanical tolerances. Current manufacturing technology is only capable of aligning printed circuit features to within +/-0.005 to 0.015 inches of other mechanical features such as edges and alignment holes. This level of precision would be inadequate to align shutoff pin 120 closely enough with aperture 117 to prevent infiltration of overcoating material into the area of interest.
It is therefore an object of the present invention to provide a printed circuit spark gap that has superior resistance to contamination caused during manufacture of the printed circuit board, and that may be easily and inexpensively manufactured.